Lighting control circuits are commonly used with electronic dimming ballasts and driver circuits to control the output of one or more lamps. Conventional lighting control circuits typically employ either an open-loop or closed-loop control methodology. A defining characteristic of an open-loop control system is that the control system does not use feedback to determine whether its output has achieved the desired goal. Instead, the open-loop controller uses a model or algorithm to determine an output based on an input. An example of open-loop lighting control is activating a lamp based on a timer. Another example of open-loop lighting control is adjusting the intensity of a lamp located inside a house based on the level of ambient light outside the house. Because an open-loop control system does not monitor the resulting illumination, the open-loop control system cannot adjust its output to compensate for any discrepancies between the desired lighting and the actual lighting.
Contrary to open-loop control systems, closed-loop control systems typically utilize some form of feedback. A closed-loop lighting control system may employ a light sensor for measuring a characteristic (e.g., intensity) of light in an area. The closed-loop control system can then adjust the output of a lamp based on the measured characteristic. For example, a closed-loop lighting system for controlling the intensity of light in a room may sample the intensity at predetermined intervals. If the sampled intensity is less than the desired intensity, the closed-loop control system can increase the output of a lamp to compensate.
Closed-loop lighting control systems are useful in simple applications that require a single controllable variable. However, closed-loop control systems are not practical for more complex systems that desire to control multiple parameters. In such complex cases, closed-loop control systems often become unstable and oscillate their outputs in an attempt to control each of the variables. Furthermore, these multi-parameter closed-loop control systems can be difficult and complex to configure.